My mother suffers from severe depression and has been hospitalised for long periods, ever since I was 13 years old. I have spent many, many Christmas days visiting secure psychiatric units, and my DM usually just spends the time staring into the distance or crying. It always makes me feel awful - awful that she feels so depressed, and awful (especially as a child) that me visiting didn't seem to brighten her day at all.
Since having DC myself, I no longer visit on Christmas day. My own childhood Christmases were incredibly hard because of these visits, and I would never let my children be affected in the same way.
I understand the guilt (and feel it myself), but the joy of our DC (who are now teenagers) having wonderful a wonderful Christmas far outweighs this for me.
My relationship with my DM is close, but I won't let my DC be affected the way my siblings and I were. I will call and speak to her on the phone, and if (by some Christmas miracle!) she's in the mood for a FaceTime, then that's what we'll do.
It's a very personal decision, and what works for me may not work for you, but you have my sympathy, it's incredibly tough. 